OWA asking users to change password; accounts are not expired

I recently updated one of the CAS boxes to SP1 RU 4v2 (the other is still on RU2). 

Since then, users trying to access OWA via Internet Explorer basic authentication are getting prompted to change their password even though their account has not expired. 

What kinds of things would cause that kind of behavior? Is it an IIS issue?

Thanks.